1. Field of the Invention
The present invention relates to an ultrasound imaging system. In particular, the invention relates to an ultrasound imaging system based on a pulse compression technique using a spread spectrum signal and a FIR filter having an efficient hardware structure.
2. Description of the Related Art
Conventionally, a medical ultrasound imaging system obtains information about a human body by transmitting short ultrasound pulses into the body and receiving a signal reflected from inside the body. FIG. 1 shows a block diagram of a conventional short-pulse ultrasound imaging system 100, which comprises a transducer array 1 having a plurality of transducers, a pulser 11, a TX(transmission) focus delay memory 14, a TX/RX(receiving) switch 21, a receiver 31, a beamformer 37, an RX focus delay adjuster 36, a signal processor 41 and a scan converter 42.
Specifically, a delay pattern of ultrasound pulses to be transmitted into an object, e.g., a human body, from the transducer array 1 is first stored in the TX focus delay memory 14. Thereafter, a binary sequence corresponding to the delay pattern stored in the TX focus delay memory 14 is generated and provided to the pulser 11.
As a method of determining the delay pattern for each of the transducers, a fixed-focusing technique is commonly used, which focuses the energies of the ultrasound pulses on a predetermined point inside the human body. Recently, as one of efforts to resolve the problem of limited resolution due to the fixed-focusing transmission compared to dynamic focusing receiving, a synthetic aperture technique has been studied. With the synthetic aperture technique, one or more transducers can be used for transmitting ultrasound pulses and bi-directional dynamic focusing is possible for both the transmitting and receiving pulses. By using the synthetic aperture technique, the resolution can be improved while SNR (signal-to-noise ratio) is decreased.
The pulser 11 is a bipolar pulser, which supplies an amplified signal (e.g., +80 or xe2x88x9280 volt) to the transducer array 1 in response to the binary sequence from the TX focus delay memory 14. The voltage output of the pulser 11, having a predetermined amplitude, is applied to each transducer of the transducer array 1 at a time determined by the delay pattern.
The transducer array 1 transmits the ultrasound pulses, in response to the output voltage of the pulser 11, into the object. A portion of the transducers in the transducer array 1 may selectively be used for one time transmission even if the transducer array 1 includes N, e.g., 128, transducers. For example, only 64 transducers within an aperture may be utilized for transmitting the ultrasound pulse at one time.
After transmitting the ultrasound pulses into the body, the transducer array 1 receives a pulse signal that is reflected from the body.
The TX/RX switch 21 acts as a duplexer for isolating the receiver 31 from the pulser 11 to protect the high voltage output from being applied to the receiver 31. The switch 21 connects the transducer array 1 to the pulser 11 during the transmission mode and to the receiver 31 during the reception mode.
The receiver 31 includes a pre-amplifier for amplifying the received signal, a TGC (time gain compensator) for compensating the attenuation during propagation of the ultrasound pulses and an analog-to-digital converter for converting the amplified received signal to a corresponding digital signal.
The beamformer 37 performs RX focusing for the corresponding digital signal that is provided from the receiver 31 in accordance with the delay pattern from the RX focus delay adjuster 36.
The signal processor 41 performs the signal processing such as envelope detection, log compensation to produce a B-mode image signal.
The scan converter 42 converts the B-mode image signal to a signal, which can be represented on a display device (not shown).
Due to the decrease in power of the ultrasound pulse during the propagation into highly attenuating medium such as rubber, soft tissue and the like, the short-pulse imaging system may not obtain correct information for a target object deep inside the body.
Since the medical ultrasound imaging system 100 may cause damage to the body if it increases the peak voltage of the transmitted short pulses, the power of the received signal cannot be increased by increasing the power of transmission pulse.
On the other hand, a pulse compression technique that is used in a radar apparatus is capable of improving the SNR of the ultrasound imaging system by increasing the average power of the transmitted pulse instead of increasing the peak voltage thereof. In an imaging system using such a pulse compression technique, generally, a long-duration waveform signal (xe2x80x9clong pulsexe2x80x9d) instead of the short pulse is transmitted to the body to increase the SNR.
In the medical imaging system 100 using the conventional short pulse, the image resolution in the ultrasound propagation direction depends on the impulse response of the ultrasound transducer which is selected and used due to the use of short pulses with a high voltage. However, in the imaging system using the pulse compression technique, the image resolution is determined by the convolution of the ultrasound transducer and the long pulse.
In an imaging system using the pulse compression technique, by using a pulse compressor having a FIR (Finite Impulse Response) filter at the ultrasound receiver, it is capable of effectively increasing the SNR by transmitting the long pulse signal having a lower voltage than the peak voltage in the short pulse technique.
In the ultrasound imaging system using the long pulse signal, the system performance is known to depend on characteristics of the long pulse signal used therein. In particular, the image quality is based on the relationship between the frequency characteristics of the long pulse signal and the ultrasound transducer. The system performance also depends on how the pulse compressor or the FIR filter is implemented.
Further, since the pulse compressor should be used per each channel for dynamic RX focusing, hardware complexity of the receiving part of the system depends on a structure of the pulse compressor.
In the ultrasound imaging system using the long pulse signal, a spread spectrum signal, e.g., a chirp signal (a linear frequency modulation signal), can be used as the long pulse signal. Particularly, The chirp signal has frequency characteristic that matches with the spectrum of the transducer of the ultrasound imaging system having a limited bandwidth. The chirp signal after passing the conventional FIR filter also has its peak side lobes that are xe2x88x9213 dB below its main lobe. However, the conventional spread spectrum signal is not suitable for use in the medical ultrasound imaging system because side lobes of the spectrum of the output signal from the pulse compressor should be xe2x88x9250 dB or more below the main lobe, to be used in the medical imaging system.
It is, therefore, a primary objective of the present invention to provide an ultrasound imaging method and system based on a pulse compression technique that uses a spread spectrum signal having tolerable side lobes.
Another objective of the present invention is to provide a FIR filter having an efficient structure and a method of determining the coefficients of the FIR filter for use in the ultrasound imaging system using the spread spectrum signal.
In accordance with one aspect of the present invention, there is provided an ultrasound imaging method for forming an image of an object using signals reflected from the object after transmitting an ultrasound pulse to the object, comprising the steps of (a) converting a predetermined first spread spectrum signal to the ultrasound signal at one or more transducers and transmitting the ultrasound signal to the object, (b) performing pulse compression on a reflected signal of the ultrasound signal reflected from the object to form a pulse compressed signal, and (c) processing the pulse compressed signal to produce a receive-focused signal and generating the image of the object from the receive-focused signal.
In accordance with another aspect of the present invention, there is provided an ultrasound imaging apparatus for forming an image of an object using a signal reflected from the object and received after transmitting an ultrasound signal to the object, comprising one or more transducers for transmitting the ultrasound signal to the object in response to the predetermined spread spectrum signal, a receiving unit for receiving the reflected signal of the transmitted ultrasound signal from the object, a pulse compression unit for performing pulse compression on the reflected signal to produce a pulse-compressed signal, and an image forming unit for producing a receive-focused signal by using the pulse-compressed signal to form the image of the object.